# registerForTraitChanges(_:action:) Registers a list of traits to observe, and calls a method on the receiving object when one of the observed traits changes. ``` @discardableResult @MainActor func registerForTraitChanges(_ traits: [UITrait], action: Selector) -> any UITraitChangeRegistration ``` ## Parameters `traits` An array of traits to observe for changes. `action` A selector identifying the method the system calls when one of the registered trait changes. ## Return Value An opaque token you can use to stop observing trait changes by passing to [`unregisterForTraitChanges(_:)`](/documentation/UIKit/UITraitChangeObservable-67e94/unregisterForTraitChanges(_:)). You don’t have to unregister your observations, and you can safely ignore this value. ## Discussion This is a convenience method for [`registerForTraitChanges(_:target:action:)`](/documentation/UIKit/UITraitChangeObservable-67e94/registerForTraitChanges(_:target:action:)) when the object receiving the registration is the target of the action. For example, when you register for changes on `self`, the `target` is `self`. The following example calls [`setNeedsLayout()`](/documentation/UIKit/UIView/setNeedsLayout()) in response to changes to size traits: ```swift let sizeTraits: [UITrait] = [UITraitVerticalSizeClass.self, UITraitHorizontalSizeClass.self] // Register for size class changes on self, and invalidate the layout in response to changes. registerForTraitChanges(sizeTraits, action: #selector(UIView.setNeedsLayout)) ``` --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)